New crisis and old mysteries: Resolving cosmic tensions to reveal the dark sector

2026-07-22

It has recently been established that the current expansion rate of the universe (i.e, the Hubble constant) as measured by local probes is discrepant at 5σ with that predicted in the standard Λ cold dark matter (ΛCDM) model calibrated onto cosmic microwave background (CMB) data. Additionally, analyses of the latest galaxy weak lensing surveys find an amplitude of density fluctuations on large scales (the S8 parameter) that is about 2−3σ lower than what is predicted in ΛCDM. This growing “crisis” is potentially the first indication of a departure from the ΛCDM model, giving hopes to access the fundamental properties of the mysterious dark matter and dark energy that pervade the universe. On the one hand, I have shown with my colleagues that the “Hubble tension” could indicate the presence of new physics in the era pre-recombination, potentially connected to new dark energy properties at early time. On the other hand, the “S8 tension” can be connected to new dark matter properties, and in particular could indicate that dark matter is not stable on cosmological timescales, or interacts with some new light species. Yet, none of the suggested models have been robustly detected nor are they able to explain simultaneously the Hubble and S8 tensions. With the support of the ERC, I will build a group that will develop public tools to firmly identify the new physics mechanism required to explain these cosmic tensions, assess the observational consequences for a wide variety of observables, and the implications for the nature of dark matter and dark energy.

France

2025-12-15